Windley Key, located near Islamorada, is one of the highest in a chain of islands formed over the last 1.8 million years.  A quarry on the key supplied limestone for rock fill during the construction of the railway to Key West.  Later, rock was cut and polished for use as decorative stone.

The Florida peninsula sits atop the eastern edge of a wide, flat Florida Platform.  This shallow platform is formed of sedimentary limestone, which is sedimentary rock composed of the tests and remains of marine plants and animals.  About 1.8 million years ago a shallow sea covered the southern portion of the Florida peninsula.  Corals became established in this shallow sea along the rim of the Florida Platform.  During this time, earth’s climate had cooled, resulting in a series of glaciations called the Pleistocene “Ice Ages”.  Over the next couple of million years sea levels fluctuated, at times dropping about 400 feet lower than present (80,000 year ago).  Corals continued to grow, forming reefs that grew upward during times sea level was higher and retreating to lower depths as sea levels fell.  This formed the Key Largo Limestone, which consists primarily of the skeletal remains of corals and invertebrate shells.  Key Largo limestone forms the foundation of the upper and middle keys.

About 120,000 years ago sea levels rose to 20 feet above the present level.  Currents created a series of sandy shoals that accumulated behind the growing coral reefs.  One series of banks covered the southwestern end of the coral reefs.  These banks formed the Lower Keys.

The last change in sea level occurred about 3,000 year ago and formed the present series of reefs and islands (Keys).